fied. (See text.)

The largest of the simple forms of sense organs are bodies visible to the
naked eye and called, from their discoverer Pacini, the _Pacinian
corpuscles_. They lie along the course of nerves in many parts of the
body, and have the general form of grains of wheat. (See Practical Work.)
The Pacinian corpuscles are composed of connective tissue arranged in
separate layers around a narrow central cavity called the core (Fig. 145).
Within the core is the termination of a large nerve fiber. These
corpuscles are found in the connective tissue beneath the skin, along
tendons, around joints, and among the organs of the abdominal cavity.

[Fig. 145]

Fig. 145--*Pacinian corpuscle*, magnified. _A._ Medullated nerve fiber.
_B._ Axis cylinder terminating in small bulb at _C._ _D._ Concentric
layers of connective tissue. _E._ Inner bulb.

The simple forms of sense organs have a more or less general distribution
over the body, and are concerned in the production of at least three
special sensations. These are _touch, temperature_, and the _muscular
sensation_.

*Touch*, or feeling, is perhaps the simplest of the sensations. The sense
organs employed are the touch corpuscles, and the external stimulus is
some form of pressure or impact. Pressure applied to the skin, by acting
on the fiber terminations in the corpuscles, starts the impulses that give
rise to the sensation. The touch corpuscles render the fiber terminations
so sensitive that the slightest pressure is able to arouse sensations of
touch. It is found that _a change of pressure_, rather than pressure that
is constant, is the active stimulus. That all parts of the skin are not
equally sensitive to pressure, and that the mind does not interpret
equally well the sensations from different parts, are facts easily
demonstrated by experiment. (See Practical Work.)

*The Temperature Sensation.*--Temperature sensations, like those of touch,
are limited almost entirely to the skin. They are of two kinds, and are
designated as _heat_ sensations and as _cold_ sensations. Whether the
sense organs for temperature are different from those of touch is not
known. It is known, however, that the same corpuscles do not respond alike
to heat, cold, and pressure.

_A Change of Temperature_, rather than any specific degree of heat or
cold, is the active temperature stimulus. The sensation of warmth is
obtained when th